Methionine kinases are classified into two categories: tyrosine kinases and serine/serine-threonine protein kinases.
Among protein kinases, it is generally believed that the tyrosine protein kinases have played an important role in a number of cell functions, in particular, participating in cell signaling. A number of research findings indicate that the tyrosine protein kinases have played an indispensable important role in cell proliferation, canceration and fission process.
Tyrosine protein kinases are classified into two categories: receptor and non-receptor tyrosine protein kinases. Receptor tyrosine protein kinases generally consist of those that are extracellular, across the cell membrane and intracellular, while non-receptor tyrosine protein kinases are completely intracellular.
So far, approximately 20 families of receptor tyrosine protein kinases have been found, among which, HER subfamily includes EGFR, HER2, HER3 and HER4, and the ligands binding to the receptors within this subfamily includes epidermal growth factor, TGF-β, HB-EGF, etc. Another subfamily is the tyrosine protein kinases associated with insulin receptors, including INS-R, IGF-IR, and IR-R. PDGF subfamily comprises PDGFRα, PDGFRβ, CSFIR, C-kit and FLK-II. In addition, there is another class of FLK subfamily comprising KDR, FLK-1, FLK-4, and Flt-1. Due to the structural and functional similarity, the two subfamilies—PFDG and FLK are usually designated into one subfamily.
There are many subfamilies for non-cell tyrosine protein kinases, such as Abl, Src, Frk, Btk, Csk, and ZAP-70, etc. Each subfamily can be further classified into different sub-subfamilies. For example, Src is one of the largest subfamilies and includes Src, Yes, Fyn, Lyn, Lck, Blk, Hck, Fgt, and Yrk. This subfamily is extensively considered to be closely related to tumor formation.
The increase in the activity of tyrosine protein kinases like Abl is directly associated with the chronic myeloid leukemia (CML) in blood. During the research on the pathogenesis of CML, Nowell et al reported the discovery of the abnormal minisome named Ph1 chromosome, presently referred to as Ph chromosome, in the cells of two CML patients for the first time in Philadelphia, USA in 1960. Along with the development of molecular biology, the molecular characteristics of Ph chromosome and its role in pathogenesis are increasingly clear. That is resulted from the translocation of No. 9 and No. 22 chromosomes, wherein the protooncogene c-ABL is broken down at the end of the long arm of No. 9 chromosome (q34) and fused with the breakpoint cluster region (M-bCR gene) at the end of long arm of No. 22 chromosome (q11). C-ABL protooncogene is 230 kb long, normally encodes a 145 kD protein with tyrosine kinase activity, and functions to regulate the expression of growth factor receptors (colony stimulating factor receptor, platelet derived growth factor, and epidermal growth factor). The breakpoint of No. 9 chromosome is generally located at the 5/end of the second exon of c-ABL gene. The second exon and its subsequent sequence are translocated to a breakpoint (BCR gene) cluster region of No. 22 chromosome having unknown functions, and spliced into a new fusion gene (Bcr-Abl gene), the latter translates a Bcr-Abl fusion protein product P210 (Bcr-Abl protein) the molecular weight of which is 21.0 kD. P210 demonstrates abnormal tyrosine kinase activity as compared with P145 encoded by c-ABL. Relevant research has already indicated that pathogenesis of CML is closely related to Bcr-Abl fusion gene and its translation product Bcr-Abl protein can facilitate cell proliferation and inhibit cell apoptosis. After self-phosphorylation, Bcr-Abl protein (P210) will provide binding sites for a series of adapter protein molecules such as Grb-2, SHC, CRKL, etc., so as to activate RAS (MAPK) signal pathway or JAK/STAT signal pathway and up-regulate the expression of intranuclear gene c-myc, bcl-2, c-fos, etc. The abnormalities of these signal pathways lead to canceration, abnormal proliferation, differentiation and inhibition of apoptosis, of bone-marrow precursor cells. Thus it can be seen that the existence of Bcr-Abl fusion gene and its chimeric transcript is an extremely important pathogenic factor and very clear prognostic marker of CML, i.e. Bcr-Abl gene is an important target gene to cure CML.
Imatinib (STI571) is a tyrosine kinase specific inhibitor marketed by Novartis Corporation. Imatinib inhibits the self-phosphorylation and substrate phosphorylation of Bcr-Abl protein by occupying the ATP binding site of Bcr-Abl fusion protein, so as to inhibit Bcr-Abl positive cell proliferation or cause apoptosis. Recent researches demonstrate that imatinib not only inhibits the activity of Bcr-Abl kinase, but also inhibits tyrosine kinase activities of stem cell factor receptor (KIT), platelet derived growth factor receptor A (PDGFR-A), and platelet derived growth factor receptor B (PDGFR-B). As a result, it is also used for the treatment of tumor such as gastrointestinal stromal tumors.
Along with the generalization of this drug in clinical applications, however, drug resistance against imatinib has occurred in increasing cases. There are a wide variety of drug resistance mechanisms against imatinib, (1) Bcr-Abl gene mutation; (2) the over-expression of Bcr-Abl protein in CML cell exceeds the competitive binding capacity of imatinib; (3) the reduction in the concentration of intracellular imatinib may be related to the increase of the expression level of glycoprotein (P-gP) on tumor cell membrane. In fact, more than 50% of drug resistance is attributed to one or more amino acid mutations of Bcr-Abl fusion protein, leading to the inability of imatinib to bind to ATB binding site of the fusion protein. One of the effective ways for overcoming the drug resistance against the existing drugs is to develop tyrosine protein kinase inhibitor with a new structure.